12 LECTURES TO SCIENCE TEACHERS. 



use a lens (G) to form an image of the cell on the screen. 

 This being arranged, I dip the glass plate into the solution. 

 You see at first not a sign of any image, but only a semi- 

 opaque film. But now we see a darkening in parts, and a 

 picture is gradually appearing. It gains intensity, and now 

 it is perfect in all its details. We will withdraw it and 

 wash it, and then treat it with a little potassium cyanide, 

 which is also a solvent of many of the compounds of silver. 

 After washing again we will place it in the lantern and throw 

 the finished picture on the screen. 



Here then we have a picture produced by the collodion 

 process. You will have noticed that this time the image 

 was brought out by a solution of ferrous sulphate and not of 

 gallic acid. Ferrous sulphate is a greedy absorber of oxygen, 

 and therefore is effective in causing a reduction of silver from 

 the nitrate, with which the plate was impregnated before 

 placing in the cell. 



We are now in a position, I think, to make an experiment 

 which I promised at an earlier part of the lecture, viz., to 

 prove to you that silver iodide is unaltered by light unless it 

 has some iodine-absorber present with it. In my hand I hold 

 a glass plate to which is adhering a collodion film containing 

 pure silver iodide and having no excess of silver nitrate. 

 After placing it in the direct rays of the beam from the electric 

 light, I apply a solution of pyrogallic acid and silver nitrate 

 to it, and there is no change apparent. Taking a similarly 

 prepared plate, I apply a small square piece of silver leaf to 

 it, brushing it well on to the film. With this camel's hair 

 brush on another portion I brush a solution of tannin in 

 alcohol, and after warming the glass through its back to 

 cause desiccation, Iwill expose it for half a minute to the light, 

 behind a negative. We will develop it in the same way as 

 our last picture, using a solution of pyrogallic acid, however, 

 instead of ferrous sulphate. Notice the result only those 

 portions of the negative appear on our plate which have been 

 coated with the silver or have received the wash of tannin. 

 After fixing our picture I throw it on the screen, and we see 

 our results more perfectly. The transparent parts through 

 which the light passes show where the actinic rays did not 

 affect the silver iodide. 



You will notice that the part A is somewhat fainter than 

 B. The former you will recollect was the part on which was 



